All buggy and truggy tires are close to the same height since the rules only allow tires to be so tall/wide. So picking a gear ratio is more of a matter of experimenting. I'd say there's more to it than just tire size. It's more of a matter of the size of the track, available traction and whether you need top speed or accelleration.

in nitro, simply judge by ur straight and how car reacts. if ur pullin 3/4s the way down, ur pretty close. only thing that could change that is if ur track is very open and high speed, less obstacles (in which case u might want more gear), or if its really tight, twisty, littered w/obstacles in close quarters...in which case u might want less gear.